NMR Relaxation Times of Benzene Adsorbed on Charcoal: Molecular Rotation and Diffusion

Abstract

NMR relaxation times have been determined as a function of temperature and composition for the protons of benzene adsorbed on charcoal. These relaxation times have been interpreted to give the following conclusions. The benzene molecule rotates more rapidly in a plane perpendicular to its sixfold axis than it does about other axes. The average enthalpy of activation for surface diffusion lies in the range 5–7 kcal/mole. At any composition, surface diffusion is characterized by a distribution of activation enthalpies with a standard deviation of about 1 kcal/mole. There are close to 1020 unpaired electronic spin/cm3. Some evidence for high-energy adsorption sites is presented.